It's not possible to block URLs from an add-on. If you think that it can be done, you're mistaken and what you're seeing is a redirect to an internal page. Some of these add-ons supply a blank page, some display a configuration page... but they all display some sort of page. Blocking a URL would stop the browser from loading anything at all. Which is what is being asked when a person is asking for a site to be blocked. A request to a URL behind one of these fake-blockers will still be seen by the server that the end user wished to block. The User's IP (or VPN IP) will still be logged on the host server.